A variety of methods exist to apply coatings, such as thin films, to substrates, such as glass. Generally, sputtering is a technique for forming a thin film on a substrate. Sputtering techniques include diode sputtering, triode sputtering, and magnetron sputtering.
Magnetron sputtering has become a widely-used sputtering technique. Films formed by sputtering can be important for numerous devices, such as semiconductors and window glass. Films, created by these processes, are typically formed with metallic materials such as silver, aluminum, gold, and tungsten, or dielectric materials such as zinc oxide, tin oxide, titanium oxide, silicon oxide, silicon nitride, and titanium nitride.
Magnetron sputtering involves providing a target, including or formed of a metal or dielectric material, and exposing the target to a plasma in a deposition chamber. Ions formed in the plasma are generally accelerated toward the target due to the presence of an electric field. Momentum from this ion bombardment is transferred to the target's surface, thereby causing atoms of the target to gain enough energy to leave the surface of the target. Some of the atoms leaving the surface of the target in this manner are, in turn, deposited on a substrate being passed proximate to the target, thereby providing a coating layer on the substrate.
The gas used to form the plasma can be an inert, non-reacting gas, such as argon. Alternately, or additionally, reactive gases, such as nitrogen or oxygen, can be used to form the plasma. Reactive gases can combine with sputtered atoms during the formation of the sputtered coating. Deposition of reacted compounds, such as zinc oxide, tin oxide, etc., can be achieved in this manner.
To improve the efficiency of the sputtering process (i.e., to improve sputtering rate), the number of available ions can be raised by increasing the density of the plasma. To obtain a high-density plasma, an electric field and a magnetic field can be used together to produce a resultant force on electrons that tends to keep the electrons in a region near the surface of the target (i.e., the “plasma-containing region,” or “confinement region”). The resultant force on electrons in such a region is governed by the vector cross product of the electric and magnetic fields (the “E×B” drift path). For example, a magnetic field can be formed such that the magnetic lines of flux are in a direction that is generally parallel to the surface of the target. In turn, an electric field can be provided (e.g., by applying a voltage to the target) to accelerate electrons in a direction perpendicular to the surface of the target. The resultant force on the ions is defined by the “E×B” drift path and is in a direction perpendicular to both the electric and magnetic fields, governed by the “right hand rule.” This force on the electrons results in “electron drift paths,” which can be used to keep the electrons near the surface of the target, where they can collide with other neutral atoms or molecules (from the plasma or sputtered atoms from the target), thereby causing further ionizations and increasing the sputtering rate.
The magnetic fields used in sputtering magnetrons are typically provided by placing one or more magnets behind the target to help shape the plasma and focus the plasma in an area adjacent the target's surface, or confinement region. In turn, the magnetic field lines are found to penetrate through the target and form arcs over the target surface such that the magnetic field lines are substantially parallel to the target surface. The plasma can be concentrated near the surface of the target by wrapping and joining the magnetic field lines upon themselves to form a closed-loop “racetrack” pattern. This can be done, for example, by using appropriately sized and shaped magnetic elements. An exemplary “planar magnetron” configuration is depicted in FIGS. 1(a) and 1(b), illustrating the formation of racetrack-shaped plasma-containing electron drift paths.
Planar magnetrons of such configuration tend to facilitate racetrack-shaped grooves being eroded into the targets, with such grooves resulting from continued sputtering in a racetrack pattern that is largely static relative to the target. Erosion is generally found to be strongest near the center of the path formed by the magnets (due to the increased confinement of plasma in this area), which tends to create a “V-shaped” racetrack groove in the target surface. As the groove deepens, it tends to have an adverse effect not only on the sputtering rate, but also on the uniformity of the film deposition on the substrate. As a result, the utilization of target material is typically quite low for planar magnetrons, with target utilization in some cases falling in the range of about 15% to 30% of total target volume.
In light of the above, cylindrical magnetron target assemblies have gained popularity in the industry. In such assemblies, the target material is often tube shaped, forming a longitudinal cavity within which a magnet assembly is located. The cylindrical target is adapted to be rotated about its longitudinal axis. In contrast, the magnet assembly within the target does not typically rotate about this axis; rather, it is commonly held in a fixed position relative to the rotating cylindrical target. The magnets in a cylindrical magnetron typically form narrow racetracks extending substantially the length of the cylindrical target. By rotating the cylindrical target relative to the magnet assembly, these types of magnetrons help address the problem of low target material utilization encountered with planar magnetron target assemblies by lessening the effects of racetrack grooving. In particular, the degree of surface erosion grooving is so reduced that target utilization of greater than 80% can generally be achieved.
Although the use of cylindrical magnetron targets has resulted in improved target utilization, it has been difficult to simultaneously optimize target utilization and sputtering uniformity.
The present invention addresses these and other problems.